1. Field of the Invention
The present invention relates to an inspection method of distortion (a magnification error, a distortion of a projected image, and the like) characteristics of a projection optical system attached to a projection exposure apparatus, which is used in the manufacture of, e.g., a semiconductor element, a liquid crystal display element, a thin-film magnetic head, or the like in a photolithography process.
2. Related Background Art
In recent years, in the manufacture of a semiconductor element, a liquid crystal display element, a thin-film magnetic head, or the like in a photolithography process, a projection exposure apparatus for exposing a pattern on a photomask or a reticle (to be referred to as a "reticle" hereinafter) on a photosensitive substrate via a projection optical system is used. The imaging characteristics required for the projection optical system of such a projection exposure apparatus have a very strict allowable range. Of the imaging characteristics, especially, distortion characteristics (imaging characteristics including a magnification error and a distortion of a projected image) of the projection optical system are adjusted to be optimized in a state wherein the projection optical system is assembled in the projection exposure apparatus. In this case, the distortion characteristics must be measured. Conventionally, the following measurement method is used for this purpose.
(A) Japanese Patent Publication No. 63-38697
In a method disclosed in this reference, a main vernier (main scale) and a sub vernier (sub scale) formed on a test reticle are printed to overlap each other on a resist layer of a photosensitive substrate (e.g., a dummy wafer), and a relative displacement between the resist patterns (main and sub scales) after development is visually inspected. More specifically, the main and sub vernier patterns are formed adjacent to each other at each of the central point of the test reticle and a plurality of points to be measured in a pattern formation region. Upon inspection, the entire surface of the reticle is exposed at a predetermined position on the photosensitive substrate, and thereafter, exposure is repeated while sequentially moving the photosensitive substrate, so that the projected image of the main vernier pattern at the central point overlaps the position of each of the previously exposed sub vernier patterns at the plurality of points. This movement is attained by a precision moving stage comprising a high-precision length measuring device such as a laser interferometer. Also, the moving amount is uniquely determined in correspondence with the designed interval between the central point on the reticle and each of the plurality of points. After the moving stage is precisely moved by a distance corresponding to the interval, the projected image of the main vernier pattern at the central point is exposed at the position of each sub vernier pattern previously exposed on the photosensitive substrate.
On the photosensitive substrate after development, resist images of overlapping main and sub vernier patterns are formed at a plurality of positions around the resist image of the main vernier pattern corresponding to the central point on the reticle. When the resist image of the overlapping vernier patterns is read visually (via a microscope), an overlapping error amount at that point can be obtained. If the feed operation of the moving stage is sufficiently precise, the read value (overlapping error amount) of the vernier patterns corresponds to a distortion amount at that point. Thus, by reading the resist images of the overlapping vernier patterns at the respective points, the distortion amounts (deviation amounts) at the respective points are obtained, thereby confirming the distortion characteristics over the entire projection field of the projection optical system.
(B) Japanese Laid-Open Patent Application No. 59-94032 (corresponding U.S. Pat. No. 4,629,313)
A method disclosed in this reference does not require printing onto a photosensitive substrate unlike in the above-mentioned prior art (A). For this purpose, a photoelectric sensor with a very small slit opening is arranged on a moving stage in a projection optical apparatus so as to directly photoelectrically detect a portion of a projected reticle pattern. More specifically, slit-shaped marks are formed at a plurality of designed positions on a test reticle, and the entire surface of this reticle is projected onto the moving stage by the projection optical system. The moving stage is moved, so that the image of each of the projected marks is detected by the photoelectric sensor, and the position is measured by a laser interferometer, thereby obtaining the projected position of each mark. Thereafter, the distortion characteristics of the projection optical system are calculated from the relationship between the measured mark positions and the mark arrangement on the reticle.
(C) Japanese Laid-Open Patent Application No. 63-177421 (corresponding U.S. Pat. No. 4,780,616)
In a method disclosed in this reference, a slit-shaped light-emitting mark is arranged on a moving stage in pace of the photoelectric sensor in the prior art (B), the light-emitting mark is reversely projected onto a reticle via a projection optical system to scan a plurality of marks on the reticle with the light-emitting mark image by moving the moving stage, and the amount of light transmitted through the reticle is received by a photoelectric sensor arranged in an exposure illumination optical system. The coordinate position on the moving stage, where the light-emitting mark overlaps each of the plurality of marks on the reticle is measured by a laser interferometer, and the distortion characteristics are measured on the basis of the measured coordinate positions and the mark arrangement on the reticle.
(D) Other Methods
As another method, a test reticle formed with a plurality of marks whose positions are known in advance is exposed on a photosensitive substrate via a projection optical system, the positional relationship among resist images of the formed marks after development is directly measured by a high-precision measurement device, and the measurement values are compared with the designed positions of the marks on the reticle, thereby obtaining the distortion characteristics. As a method without development, latent images of marks exposed on a resist layer of a photosensitive substrate, or mark images exposed and visualized on a photochromic layer formed on the surface of a photosensitive substrate in place of the resist layer may be detected.
The above-mentioned prior arts suffer from the following drawbacks or problems.
In the prior art (A), since the stage on which the photosensitive substrate is placed is two-dimensionally and repetitively step-moved to expose patterns to partially overlap each other, stop precision of the stage to each step-movement position, yawing precision, and the like are undesirably included in the measurement value of the distortion amount as an error. Furthermore, since the distance from the main vernier pattern at the central portion on the test reticle to the sub vernier pattern on a peripheral portion is as large as a maximum of about 50 mm, a drawing error (arrangement error) of each vernier pattern upon manufacture of the test reticle may sometimes be large, and be included in the measurement value of the distortion amount as an error. Such errors upon manufacture of the reticle may be measured in advance using another high-precision measurement device, and may be canceled if the measured errors are used as correction values upon calculation of the distortion amounts. However, an error corresponding to the precision of the correction values, in other words, corresponding to the measurement precision of the high-precision measurement device still remains.
Also, since the prior art (A) requires a large number of times of partial overlapping exposure, a relatively long period of time is required until all overlapping exposure processes are completed. The long exposure process time for measurement poses a serious problem since the measurement result is directly influenced by deterioration (drift) of alignment precision caused by stability (e.g., air fluctuations) of the laser interferometer used in position measurement of the moving stage, as well as an increase in load on an operator and a decrease in throughput.
Since the prior arts (B) and (C) do not require a development process which is indispensable in the prior art (A), the load on an operator is reduced accordingly. However, the problems of moving precision of the moving stage, stability of the laser interferometer, the arrangement errors of marks on the reticles, and the like are similarly posed as in the prior art (A). Furthermore, in the prior arts (B) and (C) as well, since the number of times of movement of the stage is large, a considerably long period of time is required until the positions of all marks (e.g., 200 marks) are measured.
Of the prior arts (D), the method for detecting latent images or the like is convenient in that the development process is omitted. However, not only improvement of detection precision of latent images, but also moving precision of the moving stage, temporal stability of the laser interferometer, arrangement errors of reticle marks, and the like remain unsolved as unavoidable problems.
As described above, in conventional distortion measurement, as problems of the exposure apparatus, step movement precision and moving precision (including unstable factors of the laser interferometer) of the moving stage are common drawbacks. Furthermore, the prior art (A) requires a very long period of time for the exposure operation, and the prior arts (B), (C), and (D) require a very long period of time for the measurement operation.